Smith has shown that when poly(A)+ RNA derived from human diploid fibroblasts at late passage at late passage is microinjected into the same cell type at early passage, the growth of the early passage cells is halted. A functionally identical activity derived from rat liver (by K. McClung and J. Smith) has a single molecular weight peak of activity on sucrose gradients. These data suggest that a single messenger RNA has the ability to shut off cell growth, and dilution experiments suggests that the message is abundant (1/100-1/1000 of total message). We plan to clone this mRNA by hybrid selection screening of a cDNA library, and then to analyze the structure and expression of the gene.